[1.10.x] Fixed #26970 -- Fixed crash with disabled ModelMultipleChoiceField.

Backport of 4e86168290 from master

tests/model_forms/tests.py

@@ -1514,6 +1514,31 @@ class ModelChoiceFieldTests(TestCase):
             ['Select a valid choice. That choice is not one of the available choices.']
         )
 
+    def test_disabled_multiplemodelchoicefield(self):
+        class ArticleForm(forms.ModelForm):
+            categories = forms.ModelMultipleChoiceField(Category.objects.all(), required=False)
+
+            class Meta:
+                model = Article
+                fields = ['categories']
+
+        category1 = Category.objects.create(name='cat1')
+        category2 = Category.objects.create(name='cat2')
+        article = Article.objects.create(
+            pub_date=datetime.date(1988, 1, 4),
+            writer=Writer.objects.create(name='Test writer'),
+        )
+        article.categories.set([category1.pk])
+
+        form = ArticleForm(data={'categories': [category2.pk]}, instance=article)
+        self.assertEqual(form.errors, {})
+        self.assertEqual([x.pk for x in form.cleaned_data['categories']], [category2.pk])
+        # Disabled fields use the value from `instance` rather than `data`.
+        form = ArticleForm(data={'categories': [category2.pk]}, instance=article)
+        form.fields['categories'].disabled = True
+        self.assertEqual(form.errors, {})
+        self.assertEqual([x.pk for x in form.cleaned_data['categories']], [category1.pk])
+
 
 class ModelMultipleChoiceFieldTests(TestCase):
     def setUp(self):